Log-periodic antennas have radiating elements arranged such that the dimensions both of the radiating elements and the spacing between them increase logarithmically from one end of the array to the other so that the ratio of element length to element spacing remains constant. Such antennas maintain a relatively constant radiation pattern over a large frequency range.
Although dipole arrays are frequently employed, it is possible to construct log-periodic antennas with slots as the radiating elements, as shown in U.S. Pat. Nos. 3,369,243 to Greiser and 3,633,207 to Ingerson, issued Feb. 13, 1968 and Jan. 4, 1972, respectively. The slot antenna is the dual of the dipole antenna having the same radiation pattern but with the polarization rotated by 90.degree. . The relationship between the radiation impedance Z.sub.s of the slot antenna and the radiation impedance Z.sub.d of the dipole antenna is: ##EQU1##
Two difficulties encountered in the design of logberiodic slot antennas are the impedance of the connection between the radiating elements and the requirement to provide phase reversal of the feed between adjacent elements. A simple dipole at resonance has an impedance of 73.OMEGA.. The log-periodic dipole antenna is a series of these dipoles connected by twin-lead lines. The twin-lead line has a slightly larger impedance, about 100.OMEGA.. A simple slot at resonance has an impedance of 487.OMEGA. which requires a "twin-slot", (co-planar guide) of 355.OMEGA.. A co-planar line of such impedance requires a very small, center conductor and is not practical. The required phase reversal between adjacent dipoles of a log-periodic dipole antenna is accomplished by twisting the interconnecting twin-lead between them, or by an equivalent arrangement. The same cannot be done for the log-periodic slot antenna, however, because it is topologically impossible to twist slots etched on a ground plane.